They could pretty much do it by having things like the 'for paying subscribers of X' attribute that work as 'also free on B' and 'also for paying subscribers of

They could pretty much do it by having things like the 'for paying subscribers of X' attribute that work as 'also free on B' and 'also for paying subscribers of B' and the bundle could (at least initially) be pretty much just the current substack UI. I'm sure if substack actually do something like this, they'll give it a little more thought than I just did, of course.